The present invention relates to a method of removing the electrostatic charge of an electrostatic latent image remaining on an electrophotographic photosensitive device which is formed by laminating a photoconductive layer and a transparent insulating layer on a conductive layer.
It is known that a copy of an image can be made by making use of an electrophotographic photosensitive device prepared by forming a photoconductive layer on a conductive layer and forming a transparent insulating layer on the photoconductive layer. In making the copy, an electrostatic latent image is formed on the photosensitive device. One of the following three methods is usually used for forming the electrostatic latent image when the photosensitive device is of P type:
(1) A method having a first step in which the photosensitive body is positively charged by a D.C. charger and, simultaneously, exposed to a light image, a second step in which the photosensitive device is negatively charged by a D.C. charger, and a third step in which the photosensitive device is wholly exposed thus forming an electrostatic latent image on the insulating layer.
(2) A method having a first step in which the photosensitive body is negatively charged by a D.C. charger, a second step in which the photosensitive device is positively charged and, at the same time, exposed to the light image, and a third step in which the photosensitive device is wholly exposed.
(3) A method having a first step in which the photosensitive body is negatively charged by a D.C. charger, a second step in which the photosensitive device is charged by an A.C. charger and, at the same time, exposed to the light image, and a third step in which the photosensitive device is wholly charged.
The electrostatic latent image formed by one of the methods explained above is then visualized by a developing agent and the thus obtained visible image is transferred to a transfer paper thus producing a copy. After the final copy of the electrostatic latent image is produced, the electrostatic lateral image has to be erased by the removal of the electrostatic charge, in advance of producing a copy of the next image.
A typical conventional method of removing the electrostatic charge is to subject the photosensitive device to an irradiation by light and, at the same time, to an A.C. charge by an A.C. corona discharge. This method, however, cannot perfectly remove the electrostatic charge. Namely, although the potential of the photosensitive device is reduced to a level near 0 V immediately after the removal of the charge, the potential is shifted to a level of 50 to 150 V due to the presence of relaxative residual charge. The level to which the potential is shifted varies depending on the position. For instance, in the portion of the photosensitive body in which a dark portion of the electrostatic latent image has been formed, the potential level is shifted to 150 V, while the portion in which the bright portion of the image has been formed exhibits a potential shift to 50 V. Therefore, when the copy of the next image is produced by using the photosensitive device from which the charges have been removed by the method described before, the density of the image is undesirably decreased and the quality of the image is impaired due to ghosting and/or fog.
In another known method of removing the electrostatic charge, it has been proposed to charge the photosensitive device by a D.C. charger to the polarity reverse to the residual potential, while subjecting the photosensitive device to an exposure over its entire area. For instance, when the level of the residual latent image is -500 V, a charging is effected by a positive corona charger and, at the same time, the whole surface is exposed to remove the electrostatic charge. In this method, the positive corona voltage is adjusted in accordance with the potential of the residual latent image such that a residual potential of about 0 V is obtained after the removal of the charge. Unfortunately, however, this method cannot perfectly remove the electrostatic charge and, in addition, cannot effect the control of residual potential in the areas in which bright portions of the image have been formed and the residual potential has been about +50 V. With this method, therefore, it is impossible to make uniform the residual potential level over the entire area of the photosensitive device. For obtaining an equal residual potential level in the area in which the bright portion of the image has been formed and in the area in which a dark portion of the image has been formed, it is necessary to employ a corona voltage of a sufficiently high level to equalize the potential level in the area where the dark portion of the image has been formed to the potential level of the area in which the bright portion of the image has been formed. Unfortunately, however, when the levels of the residual potential are equalized, the initial potential level is largely offset in one direction when the new latent image is formed, so that the control of potential levels in the bright and dark portions is disadvantageously made unstable.